Heads-up displays (also known as head-mounted displays or HMDs) allow a user to view a scene while relevant information is overlaid on the scene, so that the user looking through the heads-up display simultaneously sees the scene and the relevant information. For instance, a pilot looking through a heads-up display while landing an airplane simultaneously sees the airport ahead (the scene) through the heads-up display while the heads-up display projects information such as speed, heading and altitude (the relevant information) that the pilot needs to land the plane.
In some uses of a heads-up display it can be useful to know what part of the scene the user is viewing. One way to accomplish this is through eye-measurement technology, but existing eye-measurement technologies have some disadvantages. Some existing eye measurement technologies use separate optical paths for eye measurement and display but make the heads-up display more bulky and complex and less streamlined.
Some HMDs include compact near-eye displays positioned in front of, and close to, the user's eye. Enabling eye measurement in an HMD with a near-eye display when the camera and display are off-axis while solving both ergonomic fitting and algorithm complexity is difficult. On-axis approaches have been developed—using the same light guide for both a display and eye sensor, for instance—but not all display designs support this approach. The problem is considerably simpler when the display and eye sensor are arrange in a nearly co-axial configuration because the problem resolves to a two-dimensional solution.